The viscosity of lava is important because it determines how the lava will behave. Lavas with high viscosity are rhyolite, dacite, andesite and trachyte, with cooled basaltic lava also quite viscous; those with low viscosities are freshly erupted basalt, carbonatite and occasionally andesite. read more
No. Fire isn't matter - it's a chemical reaction between two types of matter. Lava (or, if it's below the surface of the Earth, magma) is the liquid state of rock. read more